Method of forming a high-temperature abrasion-resistant coating on a ferrous metal substrate, and resulting article

ABSTRACT

A method of forming a high-temperature abrasion-resistant hard facing or coating on a ferrous metal substrate by employing an aluminothermic reduction reaction. The resultant article has a hard facing layer containing from 2 to 8 percent boron. Hard faced composite articles made by the invented method, such as sintering machine crash decks, coke pusher ram shoes and grizzly bars are also disclosed.

This is a division, of application Ser. No. 332,987, filed Feb. 15,1973.

This invention relates to ferrous metal articles, each having a thicklayer of a high-temperature, abrasion-resistant alloy material tightlybonded to its surface. More particularly this invention relates toarticles, such as sintering machine crash decks, coke pusher ram shoes,chutes an grizzly bars, which require a wearing surface which isresistant to abrasion at high temperatures, that is, temperaturesbetween about 900° F. and 1600° F.

Many steel alloys possess the necessary strength to withstandconsiderable stress. However, these alloys are often deficient inresistance to erosion or wear when encountered in their intendedservice. To provide the proper wear resistance, these steels are oftencoated or "hard faced" with wear-resistant materials. High temperatureservice introduces such mechanical and metallurgical variables into thehard facing materials that they no longer perform satisfactorily. As aresult, only a few of the more conventional hard facing alloys aresatisfactory for such high temperature service, notably cobalt-based,nickel-based or austenitic-nickel-iron alloys. Unfortunately, these hardfacing alloys are extremely expensive and time consuming to apply.

Steels boronized by various techniques including pack cementation andchemical vapor deposition possess extremely high resistance to erosion.Coatings applied by these techniques generally are quite thin, rangingin thickness from 0.001 to 0.0001" thick. To increase the thickness ofsuch coatings requires considerable effort. Boronizing methods aredescribed in U.S. Pats. 3,029,162 and 3,622,402. These methods areextremely slow and require the use of chambered furnaces or retorts andgenerally do not lend themselves to the boronizing of large objects,such as sintering machine crash decks. Further such coatings areunsuited for high impact applications because they are too thin and areextremely brittle.

We are aware of the following prior art concerning aluminothermicwelding processes:

    ______________________________________                                        Carpenter et al. U.S. Pat.                                                                            2,515,191                                             Funk U.S. Pat.          3,264,696                                             Funk U.S. Pat.          3,396,776                                             Guntermann U.S. Pat.    3,421,570                                             ______________________________________                                    

We have discovered a method of facing crash decks and other objectssubject to extreme wear from high temperature, high abrasion uses, bymetallurgically bonding a unique ferroboron hard facing to a ferrousmetal substrate by a rapid and relatively inexpensive aluminothermicreduction (ATR) deposition method. We have found that when from about 2to about 8 weight percent of boron is present in the final outer surfaceof the composite that surface is hard and wear resistant. Unexpectedly,the wear resistance remained good at elevated temperatures, i.e., 1,000to 1,600° F.

It is the primary object of the subject invention to provide a ferrousmetal article having a wear resistant surface for use at elevatedtemperatures at which it is subject to high abrasion from impact orsliding friction.

It is another object of our invention to provide a method for makingsuch an article.

It is also an object to provide an ATR bonding method which requires nocleaning of the substrate prior to emplacement of the ATR charge, and nospecial igniter material in the charge.

It is also an object to provide an improved sintering machine crashdeck.

It is also an object to provide an improved coke pusher ram shoe.

It is also an object to provide a coke chute.

It is also an object to provide an improved grizzly bar.

In the drawings:

FIG. 1 is a sectioned elevational view of a refractory-lined perimeterand the necessary materials properly disposed for carrying out themethod of the invention.

FIG. 2 is a cross-sectioned elevational view similar to FIG. 1 takenafter the aluminothermic reduction reaction of the subject method hastaken place.

FIG. 3 is a cross-sectioned elevational view of the composite product ofour invention.

FIG. 4 is a cross-sectioned elevational view similar to FIG. 1 showingan alternative configuration for a refractory-lined perimeter.

FIG. 5 is a graph comparing the wear resistance of the ferroboron alloyof our hard facing as a function of the percentage of boron in the hardfacing.

FIG. 6 is a partially cross-sectioned side view of a coke pusher ram andshoe.

FIG. 7 is a cross-sectioned side view of an alternative coke pusher ramshoe.

FIG. 8 is a cross-section of a hard faced chute for hot granularmaterial.

FIG. 9 is an isometric view of a grizzly bar for a hot sinter breakingapplication.

As shown in FIG. 1, a steel substrate 10 is placed on a bed of sand 12,and the level of the sand is brought up to the level of the uppersurface of the substrate. A refractory-lined perimeter 14 comprising asteel exterior with flanges 16 for crane hooks and a refractory lining20, which is in this case graphite, has an interior dimension identicalto the exterior dimensions of the steel substrate. The perimeter ispositioned on the sand base 12 surrounding the substrate. Analuminothermic reduction charge or mixture 24 is placed within theperimeter on the substrate to a generally uniform depth of at leastabout 1/2-inch and up to a depth of about 12 inches. The resulting hardfacing layer 25 will have a thickness about 1/6 the average depth of theoriginal powdered charge. The charge consists of about three partspowdered iron oxide, which is preferably Fe₂ O₃ but can be Fe₃ O₄,preferably having a size at least as fine as -35 mesh, one part aluminumpowder preferably having a size between about -20 mesh and +325 mesh,and sufficient ferroboron to provide from 2 to 8% boron in the finalhard facing composition. The ferroboron is added in the form of crushedpellets, preferably having a size of -20 mesh. The composition range ofthe basic ATR charge is 65 to 85 parts iron oxide and 15 to 35 partsaluminum. Stoichiometric amounts are ordinarily used, but we haveemployed up to 5 % excess of either component with good results.

Optionally, refractory plates, such as graphite plates 26 shown in FIG.1, are placed atop the refractory perimeter. One of the plates isprovided with a hole 30 through which the charge is ignited. Theserefractory plates prevent splashing, contain the heat of reaction of thecharge and force such heat into the substrate to enhance the adherenceof the hard facing 25. The charge 24 is ignited by a convenient means,such as a welding torch inserted in the hole which ignites the fuelpowder, in this case aluminum. Other fuels that might be used instead ofaluminum are magnesium, calcium, silicon and calcium silicon alloy.These fuels may replace only a portion of the aluminum powder, ifdesired. The reaction is very exothermic which produces products havinga high degree of superheat from which the dense metal phase separatesand metallurgically bonds to the substrate 10. The less dense slag layer32 collects on top of the metal phase. After the reaction is complete,the graphite plates 26 are removed from atop the refractory perimeterand the product is insulated. Insulation (not shown) is provided byplacing a blanket of Kaowool or pouring sand on top of the slag crust.This causes the metal to solidify from the bottom and promote a sound,pore-free hard facing 25. The product is allowed to cool until the hardfacing has solidified at which time the insulation and the perimeter canbe removed. The slag 32 is removed merely by breaking it to leave aferrous metal substrate 10 having a boron-containing abrasion-resistantsurface 25. Heretofore, the surface of any substrate clad by analuminothermic reaction has been required to be cleaned as shown in FunkPatent No. 3,264,696; however, we have found surface preparation of thesubstrate to be totally unnecessary.

As can be seen by the graph of FIG. 5, when less than about 11/2% boronis present in the hard facing, it has no better wear resistance thanregular 1045 grade steel. Additional boron in the hard facing increasesthe wear resistance until the maximum wear resistance is obtained from aboron content of approximately 5.5%. The curve levels out thereafter andthere is no advantage in adding boron in any amount above 7 or 8%.Additional boron merely increases the cost without an attendant increasein wear resistance, and also makes the hard facing more brittle. Thus,while 11/2 to 8% boron is within the purview of our invention, we prefer4 to 7% boron with the optimum boron level being 5.5 to 7%.

Heretofore it has been known that diffusion of boron into ferroussurfaces enhances wear resistance by hardening the outer layer. It isalso known that a ferroboron coating can be applied to ferrous surfacesby flame or plasma spraying. Prior to our invention, however, themaximum thickness of such ferroboron coastings has been limited to about1 mil.

We form a ferroboron hard facing which consists of an iron-base matrixcontaining from about 20 to 90 volume percent of Fe₂ B, preferably 45 to80 volume percent of Fe₂ B, with the optimum range of 60 to 80 volumepercent of Fe₂ B. This hard facing has a minimum thickness of about 0.1inch and preferably is not less than 0.25 inch. We can form hard facingstwo inches thick or more by our method.

While in our preferred embodiment an iron-base matrix containing Fe₂ Bis formed on a ferrous metal substrate, we can form hard facing layersof any matrix-forming metal or alloy on most metallic substrates.Copper, tin, nickel, chromium, cobalt and molybdenum as well as brass,bronze, ferrous and non-ferrous alloys and stainless steels are allsuitable substrates.

The substrate should be preheated prior to placing it on the sand base.While the substrate can be preheated to any temperature below its fusionpoint, we prefer to preheat to within the range of 1400° to 2000° F.,with an optimum temperature of 1800° F.

An alternative perimeter configuration is shown in FIG. 4. The sides 34extend above the height of the graphite lining 20. This provides abetter seat for plates 26.

A sintering plant crash deck is formed of a composite as shown in FIG. 3which is suspended, usually at an angle, for hot sinter to fall on andslide down toward a breaker. Sintering plant operation requires suchcrash decks to withstand abrasion at temperatures generally about 1200°F. A crash deck or other composite having a hard facing comprising astainless steel matrix containing an effective amount of Fe₂ B will beboth corrosion resistant and abrasion resistant.

A coke pusher ram 38 is shown in FIG. 6, which has a ram shoe 40comprising three composite plates 41, 42 and 43, hard faced inaccordance with the invented method. Since the coke pusher ram movesalong the bottom of a hot coke oven to push out the coke, the hardfacing is on the bottom of the shoe where the wear resistance is mosturgently required.

An alternative coke pusher ram shoe 44, as shown in FIG. 7, has aferrous metal substrate 46, beveled at each end. The bottom 47 is firsthard faced, then beveled edges 48 and 49 are hard faced either in turnor simultaneously using a special perimeter.

A chute 50 for hot granulated material, such as coke or sinter, is shownin FIG. 8. Three elongated plates 52, 53 and 54 are hard-faced andwelded longitudinally to form a chute with the hard facing 56 on theinside or wearing surface.

FIG. 9 shows a grizzly bar 60 for a sintering plant rotary breaker 62.We form a hard facing 63 at the point of greatest wear, beneath thebreaker arm 64, and at 66, so the grizzly bar can be turned around todouble its useful life. Alternatively, the entire upper surface of thebar can be hard faced.

It is possible to form our hard facing on a substrate having an inclinedor curved surface by using special perimeter (or mold) configuration.

Our invention comprehends the hard facing composition comprisingexothermic reaction mixture of an exothermic fuel powder, such asaluminum or other fuels named hereinbefore, a particulate matrix-formingmaterial comprising a reducible oxide of at least one matrix-base metal,such as Fe₂ O₃ or Fe₂ O₃ and Cr₂ O₄ (which will form a stainless steelmatrix), and an effective amount of boron (boric oxide or ferroboron,preferably as FeB) sufficient to impart the desired abrasion resistanceto the resulting hard facing layer.

It can readily be seen from the foregoing that we have invented a methodfor cladding a metal article with a high-temperature, abrasion-resistanthard face. We have also invented a high-temperature, abrasion-resistantsintering machine crash deck, a high-temperature, abrasion-resistantcoke pusher ram shoe, a high-temperature, abrasion-resistant grizzlybar, and a high-temperature, abrasion-resistant chute for hot granularmaterial.

We claim:
 1. A composition mixture for exothermically depositing a hardfacing metallic layer onto a metallic substrate, consisting essentiallyof an exothermic fuel powder selected from the group consisting ofaluminum, magnesium, calcium, silicon, and mixtures thereof; aparticulate matrix forming material selected from the group consistingof Fe₂ O₃, Fe₃ O₄, Cr₂ O₃ and mixtures thereof; and a boron containingmaterial selected from the group consisting of boric oxide, ferroboronand mixtures thereof in an amount sufficient to yield from 4 to 7% boronin the hard facing metallic layer.
 2. A composition mixture according toclaim 1 in which said boron containing material is in an amountsufficient to yield from 5 to 7% boron in the hard facing metalliclayer.
 3. A composition mixture according to claim 1 in which saidexothermic fuel powder is aluminum.
 4. A composition mixture accordingto claim 2 in which 15 to 35 parts of aluminum is present with 65 to 85parts of matrix forming material.